1. Field of the Invention
The present invention relates to electronic ignition systems and more particularly to an over voltage detection circuit for use therein to protect the active components during load dump conditions.
2. Description of the Prior Art
Contemporary monolithic electronic ignition systems comprise a dwell control circuit portion and a power driver portion. The power driver portion is coupled to an external Darlington power amplifier that is connected in series between the primary of the ignition coil of the automobile and a sensing resistor which is utilized for sensing the magnitude of ignition coil current. Generally, as is known, in response to receiving timing signals generated in timed relationship to the engine operation, the dwell control portion of the electronic ignition system generates dwell control signals for turning on and off the power driver portion thereof during predetermined intervals of each firing cycle of engine operation. In turn, the power driver portion then renders the external Darlington amplifier conductive during a predetermined interval of the firing cycle operation to initiate current through the ignition coil. The sensing resistor is utilized to sense when the ignition coil current has ramped to a maximum operating level, which in some systems is equal to 6 amperes. At this level, the ignition coil current is limited by a feedback system coupled to the sensing resistor to the power driver portion of the ignition system. At the end of the particular firing cycle the dwell control circuit portion renders the power driver portion either conductive or nonconductive, accordingly, to shut off the external Darlington amplifier. This abruptly shuts off the current conducted therethrough and causes collapse of the field around the ignition coil to generate the spark for operating the engine as is generally understood.
In most contemporary monolithic electronic ignition systems the power driver portion comprises a "shunt" power driver transistor which is coupled at an external pin of the monolithic circuit via a small resistor to one of the battery terminals. The small value resistor is required by the automobile manufacturers for low battery cold temperature operating conditions and is typically 50 ohms. The collector of the shunt power driver transistor is coupled to the aforementioned external Darlington amplifier. During most of the firing cycle period the shunt power driver transistor is maintained in a conductive state by the dwell control circuitry to shunt current drive from the external Darlington amplifier. As aforedescribed, in response to the timing ignition signals, dwell control signals are then generated or, in the particular case, inhibited to render the shunt power device nonconductive which allows base current drive to be provided to the external Darlington amplifier and hence initiation of ignition coil current. Generally, a comparator is utilized to sense when the coil current has reached approximately 6 amperes to provide a signal to the shunt power driver transistor to render it conductive to limit the current drive to the external Darlington amplifier which causes the coil current to be limited at this value.
One problem with prior art "shunt" power drivers results from the fact that during most of a firing cycle the device is in a conductive state. Hence, the device is always dissipating power in the integrated circuit. Ideally, in any monolithic integrated circuit it is desirous to reduce power dissipation.
Another problem with the prior art type of ignition circuits described above relates to what is called a "load dump" condition generally specified by the automobile manufacturers. Under load dump conditions the integrated circuit and the external Darlington amplifier must withstand a high voltage condition (normally specified as 80 volts) occurring on the battery supply terminal without either circuit being overstressed or damaged. One manner in the prior art for protecting the external Darlington power amplifier under load dump conditions is to utilize an external resistive divider network connected to the battery supply terminal for sensing when the high voltage condition occurs. In this condition the shunt power driver device is rendered conductive which, as previously mentioned, renders the Darlington amplifier nonconductive to protect this device. However, in order to protect the Darlington amplifier the shunt power device will have essentially 80 volts at the collector thereof producing a collector current of approximately 1.6 amps therethrough. Hence, not only does the shunt power device have to be a large transistor, taking up a large part of the integrated circuit and reducing the number of components and devices which can be utilized thereon, but it also dissipates a large amount of power within the integrated circuit which is undesirous.
Hence, there is a need for providing over voltage protection during load dump conditions while eliminating the need for a large power device and also for reducing pwer dissipating in the integrated circuit.